This invention relates to a base station and to an interference reduction method in the base station, and in particular relates to a base station and an interference reduction method in the base station in which communication is performed with mobile terminals existing in a subordinate cell, and control is performed to reduce interference from mobile terminals existing in adjacent cells.
With rapid advances in practical use of CDMA (Code Division Multiple Access) communication systems, commercial services of wideband CDMA (W-CDMA) have been begun to enable exchange of large amounts of data, including video as well as audio and still images. Specifications for wideband CDMA systems have been established by the 3rd Generation Partnership Project (3GPP), an organization for standardization of third-generation mobile communication systems, and various specifications continue to be studied and appended, with the goal of obtaining systems capable of higher-quality services than are presently possible.
FIG. 14 is a diagram of the network of a W-CDMA system under current 3GPP specifications. The system comprises four types of nodes, which are a higher-level network (CN: Core Network) 100, radio network controllers (RNCs) 101#0 to 101#n, radio base stations (NodeB) 102#0 to 102#n, and mobile terminals (UE: User Equipment) 103. Each of the nodes 100, 101#0 to 101#n, 102#0 to 102#n are physically connected by ATM (Asynchronous Transfer Mode) transmission paths or similar (wire intervals). The radio base stations 102#0 to 102#n and the mobile terminals 103 are connected by radio signals (radio intervals). Iu is an interface between the radio network controllers 101#0 to 101#n and the core network 100; Iur is an interface between the radio network controllers 101#0 to 101#n; Iub is an interface between the radio network controllers 101#0 to 101#n and the radio base stations 102#0 to 102#n; and Uu is an interface between the radio base stations 102#0 to 102#n and the mobile terminals 103.
User data is transmitted from CN 100, which contains exchanges, servers, databases and similar, to the RNCs 101#0 and 101#1, via Iu circuits. When a destination mobile terminal UE 103 exists within a subordinate cell 104#1 of RNC 101#0, user data is transmitted from RNC 101#0 via an Iub circuit to NodeB 102#1 accommodating the cell, and is transmitted via the Uu interface to the mobile terminal UE 103.
In a mobile CDMA system comprising a plurality of cells as described above, signals transmitted by a mobile terminal to a connected base station (uplink signals or reverse-like signals) arrive at the base stations of adjacent cells also, and when the same frequency band is being used in uplinks between cells, such signals become interference signals for adjacent cells as a result. In particular, when a mobile terminal exists in a border region in which a plurality of cells overlap, the level of interference signals for adjacent cells due to transmission signals from the mobile terminal increases. FIG. 15 shows conceptually the interference transmitted to the base station of an adjacent cell by a mobile terminal within one of two cells. To facilitate the explanation, it is assumed that the number of base stations (cells) is two, and that the number of mobile terminals is three.
A mobile terminal MS1 within a cell CL1 is in communication with base station BTS1, but transmission signals from the mobile terminal MS1 also arrive at the base station BTS2 of an adjacent cell CL2 to become interference signals. Moreover, mobile terminals MS2, MS3 in cell CL2 are in communication with base station BTS2, but transmission signals from these mobile terminals MS2, MS3 arrive at the base station BTS1 of adjacent cell CL1 to become interference signals. In this case, the interference of mobile terminal MS3 existing in the border region at which cells CL1 and CL2 overlap is greater than the interference from mobile terminal MS2, which does not exist in the border region.
FIG. 16 is a conceptual diagram of received signal components at base stations BTS1, BTS2 of cells CL1, CL2 shown in FIG. 15. As shown in FIG. 16, the received signal power at BTS1 is the sum of the received signal power from mobile terminal MS1 in subordinate cell CL1, and the received signal power from mobile terminals MS2, MS3 within an adjacent cell (cell CL2). Strictly speaking, this power also includes thermal noise, but this is omitted. The received signal power in base station BTS2 is the sum of the received signal power of mobile terminals MS2, MS3 in subordinate cell CL2, and the received signal power from mobile terminal MS1 in an adjacent cell (cell CL1).
The maximum allowed received signal power in a base station is regarded as a radio uplink resource; this radio resource is limited by interference signals from adjacent cells. On the other hand, a mobile terminal in a cell governed by a base station which is receiving interference from an adjacent cell also provides interference to the adjacent cell, and limits the radio resources of the adjacent cell.
In a current 3GPP W-CDMA system, a radio network controller RNC controls the resource of a base station BTS performs call acceptance control (admission control, congestion control) based on the total received signal power of the base station; however, control is not executed so as to reduce the above-described interference signal power from adjacent cells. That is, a base station can control received signal power from mobile terminals within a subordinate cell, but cannot control interference signal power from adjacent cells.
Prior art for control of interference from adjacent base stations exists (see for example JP 2003-259414 A). This technology of the prior art has as an object the alleviation of interference received by mobile terminals, that is due to the downlink signals from other base stations in a communication system in which uplink and downlink communications use the same frequency band. That is, a mobile terminal monitors the strength and frequency of interference signals due to downlink signals transmitted from another base station, and when the interference level exceeds a threshold, notifies the communicating base station of the presence of interference, and gives further notification of information on the time of occurrence of the interference. Upon receiving this notification, the base station changes the subband (subchannel) being used in data transmission to the mobile terminal to eliminate the problem of interference received by the mobile terminal. However, when there are no empty bands, the problem cannot be resolved by the base station, and so the higher-level device relative to the base station is notified of the interference information (interference occurrence, time of interference occurrence). The higher-level device investigates base stations adjacent to the base station of interest, specifies an adjacent base station which is the origin of the occurrence of interference based on the time of interference occurrence, and causes the adjacent base station which is the origin of the interference to change the subband being used for data transmission. Upon receiving the instruction from the higher-level device, the adjacent base station changes the subband being used for downlink data transmission. If there are no empty subbands, transmission is interrupted.
However, this technology of the prior art does not reduce interference imparted to the base station of a cell of interest due to uplink signals from mobile terminals in cells adjacent to the cell of interest, and in particular from mobile terminals existing in proximity to the edge of the adjacent cells.